This invention relates to a propulsion pipe laying system for small diameter pipes.
This pipe laying system is constructed such that the earth and sand is excavated by a cutter of a pilot head of a pipe propulsion machine and the excavated earth and sand is introduced through an opening of the cutter into an earth and sand receiving chamber formed in the pilot head while pressurized water is supplied through a water supply pipe into the chamber to convert the earth and sand therein into a slurry state to thereby enable the earth and sand to be discharged through a slurry discharge pipe. The discharged slurry is sent into a receiving tank from which it is returned by a pump into a water supply tank and is then recycled. The basic idea of the propulsion-jack driven pipe laying system is disclosed in U.S. Pat. Nos. 4,020,641; 4,024,721 and 4,026,371 all assigned to common assignee of this application and contents therein are incorporated herein by reference.
When the land to be excavated contains a large volume of underground water and has a good water permeability and if the land is sandy, the underground water pressure P.sub.1 in the front area of the excavation cutter urges the external water together with earth and sand to flow into the earth and sand receiving chamber of the pilot head. Consequently, the slurry discharge quantity Q.sub.2 becomes greater than the water supply quantity Q.sub.1.
Stating in brief, if the underground water pressure P.sub.1 increases to a certain level, it becomes higher than the pressure loss through the slurry discharge pipe. Whilst, the pressure P.sub.2 within the earth and sand receiving chamber is nearly the same as the pressure loss created through the slurry discharge pipe, and therefore the underground water pressure P.sub.1 becomes higher than the pressure P.sub.2. In consequence, water accompanied by earth and sand flows into the earth and sand receiving chamber through the opening of the excavating cutter, and so the slurry discharge quantity Q.sub.2 exceeds the water supply quantity Q.sub.1.
As a result of this phenomenon, earth and sand in volume of more than the excavated space is discharged thus forming a big cavity around the excavating cutter.
Thus, the earth and sand above the excavated space tends to fall in and as a result it becomes impossible to correct downwardly the direction of movement of the pilot head thus causing its upward movement.
Another disadvantage of the conventional propulsion pipe laying system resides in that, when the pipe propulsion is suspended and the buried pipe is extended, the water supply pipe and the slurry discharge pipe installed on the side of a pit are detached from those pipes on the side of the pilot head, and therefore the underground water flows through the water supply and slurry discharge pipes into the pit. The outflow of the underground water sweeps away part of the underground earth and sand with the result that the earth and sand may deposit in the water supply and slurry discharge pipes.
In such a case, when restarting the excavation, earth and sand may clog the slurry discharge pipe, and so it is impossible to discharge slurry. Because of a high internal pressure prevailing therein, the water supply pipe can send the deposited earth and sand into the earth and sand receiving chamber, however, there may be a problem of clogging of the pipe as well.